Translational enhancers are mRNA motifs that enhance translational efficiency of encoded proteins. In advance of this research project, we had found two translational enhancer elements in plants. The initial plan of this project was to elucidate the enhancing mechanism of these motifs by use of standard recombinant DNA technology. However, halfway the research period, we found that the enhancer effects of these motifs are greatly altered by nucleotide sequences in the UTRs well as the coding region. This suggests that the enhancing effects of these motifs are not autonomous, but based on or sensitive to the complexed intra-molecular interactions of base pairing. There has been no experimental approach that applicable to such a complexed and delicate translational regulation. Conventional base substitution, insertion, and/or deletion technique may cause unexpected conformational change of mRNA molecules, resulting in unforseen base-interactions. From these, we altered the course of this project in 1999 to establish the experimental system applicable to such a complexed translational enhancement. After two years of trial and error, we succeeded in establishing the epoch-making system, with the aid of in vitro evolution theory and PCR technology. This system allows us to find translational enhancer motifs on any given mRNA molecules. Comparative analyses of the enhancer motifs thus far obtained revealed their common characteristics ; position, sequence and strength of then are different according to the presence or absence of 5' cap and 3' poly(A) tail.